1. Field of the Invention
The present invention generally relates to an optical scanning device, an image forming apparatus and an optical scanning method, and, in particular, to improvements in an optical scanning device acting as a writing optical system of an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, or the like.
2. Description of the Related Art
A digital image forming apparatus includes an optical scanning device. In the apparatus, a laser light emitted from the optical scanning device is applied onto a photosensitive body so that an electrostatic latent image is formed thereon, the electrostatic latent image is developed by a developing device by using toner, and the thus-obtained toner image is transferred and fixed onto a transfer paper. Thereby, image formation is rendered. The optical scanning device is a device which produces the laser light based on an electric image signal obtained from conversion from optical image data such as that of a reflected beam obtained from reading and scanning operation performed by a reading optical system, and emits the produced laser light onto the photosensitive body.
FIG. 1A shows a plan view of an optical scanning device (laser writing device) in the related art, FIG. 1B shows a configuration of an essential part thereof, and FIG. 1C shows a perspective view of an essential part thereof.
In FIGS. 1A and 1B, a beam emitted by an LD unit 1 is condensed in sub-scan directions (directions perpendicular to FIG. 1A) by a cylindrical lens 2. The condensed beam is used to image a latent image long along a main scan direction on or in the vicinity of a deflection reflective surface 4 of a polygon mirror after passing through a transparent parallel plate 3 provided on a window of a housing, not shown in the figure, tightly enclosing the polygon mirror (light deflector) 5.
A reflected beam (deflected beam) from the deflection reflective surface 4 passes through the parallel plate 3, is incident on a lens system part 6 (lens 6xe2x80x2 and lens 6xe2x80x3) having an fxcex8 function and a long-dimensional lens 8, respectively, and, then, is condensed toward a surface of a photosensitive body (surface to be scanned) 7. The lens system part 6 (lens 6xe2x80x2 and lens 6xe2x80x3) having the fxcex8 function and long-dimensional lens 8 act as a scanning and imaging lens.
Recently, high density image formation has been demanded. Accordingly, reduction of diameter of a beam spot formed on the surface to be scanned 7 is strongly demanded. For this purpose, complex shapes such as aspherical surfaces, special toroidal surfaces and so forth have been employed in the fxcex8 lens system part (lenses 6xe2x80x2 and 6xe2x80x3) and long-dimensional lens 8 of the scanning and imaging lens, and these lenses have been made of plastic through resin molding technique in many cases.
A lens made of plastic (referred simply to as xe2x80x98plastic-made lensxe2x80x99, hereinafter) has its rigidity increased as a result of a lens body thereof (long-dimensional lens 8) being supported by a holding frame (rib) 9, and, thereby, deformation thereof due to time elapsing is avoided. Specifically, manufacture is made such that the holding frame 9 is formed integrally with the long-dimensional lens 8 so as to enclose the four sides of the lens 8. As shown in FIGS. 1B and 1C, rib surfaces 9a which are parallel to sub-scan directions of the long-dimensional lens 8 are formed on the inner surfaces at both ends in the longitudinal directions of the holding frame 9.
The holding frame 9 is provided outside of an effective diameter (a range of the lens surface corresponding to the range through which a proper image is written on the photosensitive body which is an effective image display device writing region/range). However, the beam needs to be made to pass through the lens even outside of the effective diameter for the purpose of synchronization of each scan line. Specifically, a synchronization detecting sensor, not shown in the figure, is provided outside of the effective diameter in a main scan direction, and, thereby, a first scanning point of the deflected beam and scanning speed (time) are detected.
At this time, as also shown in FIG. 1B, the beam B applied to the outside of the effective diameter is reflected by the rib surface 9a so as to become a ghost light, is then applied to the surface to be scanned 7, and, as a result, an abnormal image may formed thereon. This is because, as the rib surfaces 9a, in the related art, are surfaces parallel to the sub-scan directions of the long-dimensional lens 8, the ghost light therefrom is applied to the surface to be scanned 7 within an effective image region thereof when the beam B applied to the outside of the effective diameter is reflected by the rib surface 9a, and, thereby, an abnormal image is formed on the surface to be scanned 7, in many cases.
The present invention has been devised in view of the above-mentioned problem, and an object of the present invention is to prevent the ghost light generated due to reflection by the rib surface of the holding frame from adversely affecting the proper image formation onto the surface to be scanned so as to render a satisfactory proper image.
According to the present invention, in an optical scanning device including a plastic-made scanning and imaging lens reinforced by a holding frame, a ghost light generated as a result of a beam deflected by a light deflector and applied to the outside of an effective diameter of the scanning and imaging lens being reflected by a rib surface of the holding frame is prevented from adversely affecting proper image formation so that a satisfactory proper image can be rendered.
Specifically, in an optical scanning device, according to the present invention, condensing a beam deflected by a light deflector, by a scanning and imaging lens toward a surface to be scanned to form a beam spot thereon, and scanning the surface to be scanned by the beam spot,
at least one lens of the scanning and imaging lens is configured so that a lens body thereof is held by a holding frame,
wherein a rib surface at an end in a longitudinal direction of the holding frame is inclined so that a ghost light generated as a result of the deflected beam being reflected by the end in the longitudinal direction of the holding frame is changed in light path in a sub-scan direction.
The rib surface is inclined as a result of being rotated about an axis parallel to an optical axis of the lens body, for example.
Accordingly, it is possible to prevent the ghost light from being applied to the effective writing range of the surface to be scanned, and, to prevent any abnormal image formed therefrom from being formed. Specifically, the ghost light does not adversely affect proper image formation, and, as a result, a sufficient proper image can be rendered.
In an optical scanning device, according to another aspect of the present invention, condensing a beam deflected by a light deflector, by a scanning and imaging lens toward a surface to be scanned to form a beam spot thereon, and scanning the surface to be scanned by the beam spot,
at least one lens of the scanning and imaging lens is configured so that a lens body thereof is held by a holding frame,
wherein a rib surface at an end in a longitudinal direction of the holding frame is inclined so that a ghost light generated as a result of the deflected beam being reflected by the holding frame is turned outside of an effective writing range in a main scan direction.
The rib surface is inclined as a result of being rotated about an axis parallel to a sub-scan direction of the scanning and imaging lens, for example.
Accordingly, it is possible to prevent the ghost light from being applied to the effective writing range of the surface to be scanned, and, to prevent any abnormal image formed therefrom from being formed. Specifically, the ghost light does not adversely affect proper image formation, and, as a result, a sufficient proper image can be rendered.
In an optical scanning device, according to another aspect of the present invention, condensing a beam deflected by a light deflector, by a scanning and imaging lens toward a surface to be scanned to form a beam spot thereon, and scanning the surface to be scanned by the beam spot,
at least one lens of the scanning and imaging lens is configured so that a lens body thereof is held by a holding frame,
wherein a rib surface at an end in a longitudinal direction of the holding frame is curved so that a ghost light generated as a result of the deflected beam being reflected by the holding frame forms a beam spot sufficiently spread on the surface to be scanned.
Thereby, the beam spot formed on the surface to be scanned acting as the photosensitive body from the ghost light is sufficiently spread, has a low energy density, and, thus, does not result in substantial exposure of the photosensitive body. Accordingly, it is possible to render a sufficient proper image.
Any of the above-mentioned lens body and holding frame may be manufactured through integral molding of a plastic material. Accordingly, it is possible to achieve an inexpensive and high-rigidity scanning optical system including the scanning and imaging lens by which the ghost light generated from the rib surface of the holding frame does not adversely affect the proper image formation.